1. Field of the Invention
This invention relates to a stimulable phosphor sheet erasing method and apparatus. This invention particularly relates to a stimulable phosphor sheet erasing method and apparatus, in which residual radiation energy remaining on a stimulable phosphor sheet is erased such that the erased stimulable phosphor sheet becomes capable of being used for a next radiation image recording operation.
2. Description of the Related Art
Radiation image recording and reproducing systems utilizing stimulable phosphors have heretofore been known. With the radiation image recording and reproducing systems utilizing the stimulable phosphors, radiation carrying image information of an object, such as a human body, is irradiated to a sheet containing a stimulable phosphor (hereinbelow referred to as the stimulable phosphor sheet), and a radiation image of the object is thus stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during the exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet is photoelectrically detected, and an image signal representing the radiation image of the object is thereby acquired.
In the radiation image recording and reproducing systems described above, after the radiation image has been read out from the stimulable phosphor sheet, erasing light having wavelengths falling within a wavelength range of visible light is irradiated to the stimulable phosphor sheet, and residual radiation energy remaining on the stimulable phosphor sheet is thereby released. The thus erased stimulable phosphor sheet is again used for the recording of a radiation image.
In such cases, it often occurs that, on the erased stimulable phosphor sheet from which the residual radiation energy has been released, part of the residual radiation energy remains even further without being released perfectly during the erasing operation. The residual radiation energy remaining even further on the stimulable phosphor sheet without being released perfectly during the erasing operation is the energy due to the radiation image, which was recorded on the stimulable phosphor sheet in the manner described above. If the energy level of the residual radiation energy remaining even further on the stimulable phosphor sheet without being released perfectly during the erasing operation is higher than a negligible level, and the stimulable phosphor sheet, on which the energy level of the residual radiation energy is higher than the negligible level, is subjected to a next radiation image recording operation for the recording of a radiation image of an object, the residual radiation energy remaining on the stimulable phosphor sheet will cause a residual image to occur in a radiation image, which is reproduced from an image signal having been read out from the stimulable phosphor sheet having been subjected to the next radiation image recording operation. The residual image constitutes noise in the reproduced radiation image.
Therefore, there have been proposed various techniques for adjusting the level of erasing energy for the erasing of the stimulable phosphor sheet such that the residual image may not appear in the reproduced radiation image. For example, techniques for adjusting the quantity of imparted erasing energy in accordance with the level of residual radiation energy remaining on a stimulable phosphor sheet have been proposed in, for example, U.S. Pat. No. 5,530,261 and Japanese Unexamined Patent Publication No 2003-295364. Also, a technique for setting the quantity of imparted erasing energy to be large or performing an additional erasing operation for a stimulable phosphor sheet in cases where a next radiation image recording operation, which is to be performed on the stimulable phosphor sheet, is a high-sensitivity radiation image recording operation has been proposed in, for example, Japanese Unexamined Patent Publication No. 10(1998)-340920. Further, a technique for adjusting erasing energy in accordance with a radiation dose, which has been delivered from a radiation source in a radiation image recording operation that has been performed most recently on a stimulable phosphor sheet, and the radiation dose, which is expected to be delivered from the radiation source in a next radiation image recording operation that is to be performed on the stimulable phosphor sheet, has been proposed in, for example, Japanese Unexamined Patent Publication No. 2002-296713.
In cases where the energy level of the residual radiation energy, which remains even further on the stimulable phosphor sheet after an erasing operation has been performed on the stimulable phosphor sheet, (hereinbelow referred to as the after-erasing radiation energy) is sufficiently lower than the energy level of radiation energy, which is expected to be stored on the stimulable phosphor sheet with the next radiation image recording operation, the residual image due to the after-erasing radiation energy is capable of being rendered imperceptible or invisible in the reproduced radiation image. Also, in cases where the quantity of imparted erasing energy is set to be large such that the energy level of the after-erasing radiation energy may become low, the problems occur in that the service life of an erasing lamp becomes short, or in that the image recording throughput rate becomes low. Specifically, in order for the energy level of the erasing energy to be set to be high, it is necessary that the light quantity of the erasing light be set to be large, or that the erasing light irradiation time be set to be long. However, in cases where the light quantity of the erasing light is set to be large, the problems occur in that the service life of the erasing lamp becomes short. Also, in cases where the erasing light irradiation time is set to be long, the image recording throughput rate becomes low. Therefore, in cases where the erasing operation is to be performed on the stimulable phosphor sheet, it is desired that an appropriate quantity of erasing energy, which quantity is not excessively large and is not insufficient, be imparted to the stimulable phosphor sheet.
It is desired that the ratio of the energy level of the after-erasing radiation energy (i.e., the energy level of the residual radiation energy remaining on the stimulable phosphor sheet at the time at which the next radiation image recording operation is to be performed on the stimulable phosphor sheet) to the energy level of the radiation energy, which is expected to be stored on the stimulable phosphor sheet with the next radiation image recording operation, falls within an appropriate range, such that the radiation image, which is reproduced from the image signal having been read out from the stimulable phosphor sheet having been subjected to the next radiation image recording operation, may be substantially free from the residual image, and such that the residual radiation energy remaining on the stimulable phosphor sheet may not be erased excessively. The appropriate range of the ratio described above will hereinbelow be referred to as the appropriate erasing ratio range.
However, with the conventional erasing techniques, the adjustment of the quantity of imparted erasing energy is not performed through sufficient consideration of the ratio of the energy level of the after-erasing radiation energy to the energy level of the radiation energy, which is expected to be stored on the stimulable phosphor sheet with the next radiation image recording operation. Therefore, with the conventional erasing techniques, it often occurs that an excessively large quantity of the erasing energy is imparted to the stimulable phosphor sheet.